Optical filtering system for a laser bar code scanner having narrow band-pass characteristics with spatially separated filtering elements

ABSTRACT

A laser bar code symbol scanner employing a narrow band-pass optical filtering system of novel construction is disclosed. A first optical filtering element is installed in the light transmission aperture of a bar code scanner housing. A second optical filtering element is installed inside the scanner housing near the light detecting element. The first and second optical filtering elements have wavelength selective properties such that taken together they cooperate to form a narrow wavelength band-pass optical filtering system which allows only transmission of light at and around a certain predetermined wavelength into the photodetector element. The present invention also hides aesthetically unappealing electro-optical components mounted in the scanner housing from plain view and the optical filtering elements of the system can be easily and inexpensively manufactured without compromising the performance of the scanner.

RELATED CASES

This Application is a Continuation of application Ser. No. 08/850,295,filed May 5, 1997 now abandoned, which is a Continuation of applicationSer. No. 08/439,224, filed May 11, 1995, now U.S. Pat. No. 5,627,359,which is a Continuation-in-Part of application Ser. No. 08/293,491 filedAug. 19, 1994, now abandoned which is a Continuation of application Ser.No. 07/761,123 filed Sep. 17, 1991, now U.S. Pat. No. 5,340,971,incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to laser scanners used inreading bar and like code symbols, and more particularly to a noveloptical filtering system for use therein, which provides improvedscanner performance, appearance and manufacturability at lower cost.

2. Brief Description of the Prior Art

Laser-based bar code symbol scanning systems have become increasinglypopular in recent times. However, despite technical advancements in theart, such systems still suffer from numerous problems that have yet tobe adequately solved.

For example, a major problem with prior art laser scanners is that asthey become more widely used in point-of-sale (POS) environments,aesthetic considerations play a greater role in their purchase decisionsby store managers considering their use at POS locations. The reason forthis is clear. Store owners invest in a great deal of time, money andartistic effort in making their stores and display counters attractiveto customers. Consequently, store owners and managers demand that laserscanning systems do not detract from the appearance of their display andcheck-out counter environments.

Another problem with prior art laser scanning systems is that the laser,mirrors, and other electro-optical components used in such systems arerevealed to customers at POS locations through optically transparentscanning windows. Consequently, the sight of rotating mirrors andswirling laser beams behind the scanning windows of prior art laserscanners, constitutes a significant source of fear to many customers.While such fears are often based on a lack of knowledge of lasers andoptics, store managers are nevertheless concerned that such fears maytranslate into customer anxiety and thus a decrease in sales.

Other problems of a more technical nature arise when using prior artlaser scanners in POS environments. In particular, typical ambientlighting levels in store environments have the potential of adverselyeffecting the signal-to-noise ratio (SNR) of laser scan data signalsdetected within prior art laser scanners. Thus, to date, a number ofdifferent optical filtering techniques have been developed for use incombating the adverse effects of ambient lighting levels on laserscanner performance. Several optical filtering techniques commonlyemployed are detailed below.

One popular filtering technique involves installing before the scannerphotodetector, a band-pass optical filter narrowly tuned to the laserwavelength. Typically, this wavelength lies in the visible region of theelectromagnetic spectrum (i.e., about 670 nanometers). This commonfiltering technique is used in the prior art laser scanning systemsdisclosed in U.S. Pat. Nos. 5,180,904; 5,015,833; 4,816,660; 4,387,297and 5,115,333. However, this approach is not without shortcomings anddrawbacks. When using this approach, store customers are typicallypermitted to see the rotating or oscillating mirrors and swirling laserbeams behind the scanning window. In addition to presenting a source ofworry for many customers, the plain view of such electro-opticalcomponents also detracts from the overall aesthetic appearance of laserscanners employing this common filtering technique.

Another prior art approach to reducing ambient light in a post-basedlaser scanners involves installing a spatial filter (i.e., a slotted oraperture plate) over the scanning window of the laser scanner.Typically, the aperture or slot pattern of the aperture plate spatiallycorresponds to the cross-sectional geometry of projected laser scanningpattern at the plane of its scanning window. This spatial filteringtechnique is used in the many prior art laser scanning systems,disclosed in U.S. Pat. Nos. 4,713,532; 4,093,865; and 4,647,143.However, this approach is not without its shortcomings and drawbacks.Such spatial filters detract from the overall appearance of the laserscanners in which they are employed. In addition, such spatial filterscannot be effectively used when the laser scanning patterns arespatially complex, as in the case of the omnidirectional projectionlaser scanner disclosed in U.S. Pat. No. 5,216,232.

Thus, there is a great need in the art for a laser scanner which solvesthe above-described problems, while overcoming the shortcomings anddrawbacks of prior art laser scanning apparatus and methodologies.

OBJECTS OF THE INVENTION

Accordingly, it is a primary object of the present invention to providea laser bar code symbol scanning system that is capable of reading barcode symbols, without the shortcomings and drawbacks of prior artdevices.

A further object of the present invention is to provide a laser bar codesymbol scanner having a novel optical filtering system which providesimproved scanner performance, appearance and manufacturability.

A further object of the present invention is to provide such a laser barcode symbol scanner, in which the wavelength-selective components of theoptical filter system are strategically installed in aspatially-separated manner in order to achieve improved scannerperformance, appearance and manufacturability, in a simple low-costmanner.

A further object of the present invention is to provide such a laser barcode symbol scanner in which the optical filtering system employedtherein inherently hides from view, unappealing electro-opticalcomponents mounted within the laser scanner housing, while rejectingunwanted spectral noise outside the narrow spectral band of the laserscanning beam.

A further object of the present invention is to provide a laser bar codesymbol scanner that satisfies the concerns of store owners and managersalike, while effectively overcoming the problems caused by highintensity ambient lighting.

These and further objects of the present invention will become apparenthereinafter and in the claims.

SUMMARY OF THE PRESENT INVENTION

In general, the laser scanner of the present invention provides asimple, low cost solution to the problems described in the Background ofthe Invention. This is achieved by strategically embodying a pair ofdiscrete optical filter elements in the housing of a laser scanner inwhich the following system components are provided; a light transmissionwindow; a laser source for producing a laser beam having a predeterminedcharacteristic wavelength; a scanning mechanism for projecting theproduced laser beam through the light transmission window, and scanningthe produced laser beam across a scanning field defined external to thehousing; a laser light focusing means for focusing laser light reflectedoff a scanned bar code symbol, and along a focused laser light returnpath within the housing; and a laser light detection means, disposedalong the focused laser light return path, for detecting the intensityof focused laser light and generating an electrical signalrepresentative thereof.

In accordance with the present invention, the first optical filterelement is installed over the light transmission aperture of the scannerhousing, and has wavelength selective properties which transmit onlylight having wavelengths from slightly below a predetermined wavelengthin the visible band of the electromagnetic spectrum (e.g., slightlybelow 670 nanometers and greater). The second optical filter element isinstalled within the housing, along the focused laser return light pathand between the light focusing means and the first optical filterelement, and transmits only light having wavelengths from slightly abovethe predetermined wavelength (e.g., slightly above 670 nanometers andgreater). Collectively, the first and second optical filter elementscooperate to form a narrow wavelength band-pass filtering systemcentered about the predetermined wavelength, thereby rejectingwavelengths outside the spectral band of the scanned laser beam and thusproviding improved signal-to-noise ratio.

As a result of this novel laser scanner construction, the wavelengthselective properties of the first optical filter element inherentlyrender it semi-transparent, and thus hide from plain view, otherwiseaesthetically unappealing electro-optical components mounted within thescanner housing. At the same time, the second optical filter element canbe made substantially smaller than the size of the light transmissionwindow over which the first optical filter element is installed, yetstill cooperate with the first optical filter element to achieve narrowwavelength band-pass filtering about the characteristic wavelength ofthe laser beam. Whereas the optical filtering properties of therelatively large first optical filter element render its manufacturerelatively easy and inexpensive, the optical filtering properties of therelatively small second optical filter element render its manufacturerelatively difficult and expensive. Thus, laser scanner construction ofthe present invention represents a significant advance in the state ofthe art in laser scanner design and construction.

In summary, the present invention provides a simple and inexpensive wayof making a laser bar code symbol scanner that satisfies the concerns ofstore owners and managers alike, while effectively overcoming theproblems caused by high intensity lighting conditions in POSenvironments.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the Objects of the Present Invention, theDetailed Description of the Illustrated Embodiments will be taken inconnection with the accompanying Drawings, wherein:

FIG. 1 is a perspective view of a laser bar code symbol reading deviceconstructed in accordance with the principles of the present invention;

FIG. 2 is a cross-sectional elevated side view along the longitudinalextent of the bar code symbol reading device of FIG. 1, showing varioushardware and software components used in realizing the illustrativeembodiment;

FIG. 2A is a cross-sectional plan view along with longitudinal extent ofthe bar code symbol reading device taken along line 2A--2A of FIG. 2,also showing the various components used in realizing the illustrativeembodiment;

FIG. 3 is schematic representation of the spectraltransmissioncharacteristics of the first and second optical filter elements employedin the laser bar code symbol reading device of the present invention,graphically illustrating how the spectral transmission characteristicsof these spatially-separated optical filter elements cooperate toproduce a narrow-band optical filter system centered about thecharacteristic wavelength of the visible laser scanning beam;

FIG. 4 is a block functional system diagram of the bar code symbolreading device of the illustrative embodiment of the present invention,illustrating the principal components of the device integrated with thecontrol system thereof;

FIG. 5 is a perspective view of alternative embodiment of the laser barcode symbol reading device of the present invention; and

FIG. 5A is a cross-sectional view of the laser bar code symbol readingdevice of FIG. 5, taken along line 5A--5A thereof.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT

For purposes of illustration, the present invention will be describedbelow with reference to the accompanying Drawings, with like structuresbeing indicated by like reference numbers.

As shown in FIG. 1, automatic bar code symbol reading system 1 of thefirst illustrative embodiment comprises an automatic hand-holdable barcode symbol reading device 2 operably associated with hand-holdable datacollection device 3, described in detail in U.S. Pat. No. 5,340,971.Operable interconnection of bar code symbol reading device 2 and datacollection device 3 is achieved by a flexible multiwire connector cord 4extending from bar code symbol device 2 and plugged directly into thedata-input communications port of the data collection device 3.

Referring to FIGS. 1 through 2A, automatic bar code symbol readingdevice 2 is shown to comprise an ultra-lightweight hand-holdable housing5 having a head portion 5A that continuously extends into a contouredhandle portion 5B. As illustrated in FIGS. 1 through 3A, the headportion of housing 5 has a transmission aperture 6 formed in an upperportion of front panel 7 and covered by plastic filter lens 69, topermit laser radiation of a predetermined band of wavelengths, to exitand enter the housing. In general, the lower portion of front panel 7Bis optically opaque, as are all other surfaces of the housing.

As illustrated in FIG. 1, bar code reading device 2 generates twodifferent fields external to the hand-holdable housing, in order tocarry out automatic bar code symbol reading according to the principlesof the present invention. Specifically, an object detection field,indicated by broken and dotted lines, is provided externally to thehousing for detecting energy reflected off an object bearing a bar code,located within the object detection field. A scan field, on the otherhand, having at least one scanning plant of essentially planar extent,is provided external to the housing for scanning an object presentwithin the scan field. Such scanning is achieved with a laser light beamso that scan data can be collected for detecting the presence of a barcode within the scan field, and subsequently reading (i.e., scanning anddecoding) the detected bar code symbol.

In general, the energy reflected off an object in the object detectionfield can be optical radiation or acoustical energy, either sensible ornon-sensible by the operator, and may be either generated by an externalambient source, or from the automatic bar code symbol reading deviceitself. In the illustrative embodiment, this energy is a beam ofinfrared light projected forwardly from transmission aperture 6 in aspatially directed fashion, preferably essentially parallel to thelongitudinal axis 9 of the head portion of the housing. In a preferredembodiment, the object detection field has a three-dimensionalvolumetric expanse spatially coincident with the transmitted infraredlight beam. This ensures that an object within the object detectionfield will be illuminated by the infrared light beam and that infraredlight reflected therefrom will be directed generally towards thetransmission aperture of the housing where it can be detected, toindicate that an object is within the object detection field.

In order to scan a bar code symbol on an object within the objectdetection field, a laser light beam having a characteristic wavelengthλc is automatically generated within the head portion of the housing andrepeatedly scanned through the transmission aperture across the scanfield. As illustrated in FIG. 1, at least a portion of the scanned laserbeam aligned with bar code on the detected object, will be reflected offthe bar code and directed back towards and through the transmissionaperture for collection, detection and subsequent processing in a mannerwhich will be described hereinafter.

To more fully appreciate the mechanisms employed in providing the objectdetection and scan fields of bar code symbol reading device 2, referenceis best made to the operative elements within the hand-holdable housing.

As shown in FIG. 4, bar code symbol reading device of the firstillustrated embodiment comprises a number of system components, namely,an object detection circuit 10, scanning means 11, photoreceivingcircuit 12, analog-to-digital (A/D) conversion circuit 13, bar codepresence detection module 14, bar code scan range detection module 15,symbol decoding module 16, data format conversion module 17, symbolcharacter data storage unit 18, and data transmission circuit 19. Inaddition, a magnetic field sensing circuit 20 is provided for detectinga housing support stand, while a manual switch 21 is provided forselecting long or short range modes of object and bar code presencedetection. As illustrated, these components are operably associated witha programmable system controller 22 which provides a great degree ofversatility in system control, capability and operation. The structure,function and advantages of this controller will be described in detailhereinafter.

In the illustrative embodiment, system controller 22, bar code presencedetection module 14, bar code scan range detection module 15, symboldecoding module 16, and data format conversion module 17 are realizedusing a single programmable device, such as a microprocessor havingaccessible program and buffer memory, and external timing means. It isunderstood, however, that any of these elements can be realized usingseparate discreet components as will be apparent to those skilled in theart.

The purpose of the object detection circuit is to determine (i.e.,detect) the presence of an object (e.g., product, document, etc.) withinthe object detection field of bar code symbol reading device 2, and inresponse thereto, automatically produce first control activation signalA₁. In turn, first control activation signal A₁ is provided as input tothe system controller which, as will be described in greater detailhereinafter, causes the device to undergo a transition to the bar codesymbol presence detection state.

As illustrated in FIG. 4, scanning means 11 comprises a light source 47which, in general, may be any source of intense light suitably selectedfor maximizing the reflectivity from the object's surface bearing thebar code symbol. In the illustrative embodiment, light source 47comprises a solid-state visible laser diode (VLD) which is driven by aconventional driver circuit 48. In the illustrative embodiment, thewavelength of laser light produced from laser diode 47 is about 670nanometers. In order to scan the laser beam output from laser diode 47over a scan field having a predetermined spatial extent in front of thehead portion of the housing, a planar scanning mirror 49 can beoscillated back and forth by a stepper motor 50 driven by a conventionaldriver circuit 51, as shown. However, it is understood that otherconventional laser scanning mechanisms may be used to practice thepresent invention.

To selectively activate laser light source 47 and scanning motor 50, thesystem controller provides laser diode enable signal E_(L) and scanningmotor enable signal E_(M) as input to driver circuits 48 and 51,respectively. When enable signal E_(L) is a logical "high" level (i.e.,E_(L) =1), a laser beam is generated, and when E_(M) is a logical highlevel the laser beam is scanned through the transmission aperture andacross the scan field.

When an object such as product bearing a bar code symbol is presentedwithin the scan field at the time of scanning, the laser beam incidentthereon will be reflected. This will produce a laser light return signalof variable intensity which represents a spatial variation of lightreflectivity characteristic of the spaced apart pattern of barscomprising the bar code symbol. Photoreceiving circuit 12 is providedfor the purpose of detecting at least a portion of laser light ofvariable intensity, which is reflected off the object and bar codesymbol within the scan field, and subsequently focused along a focusedlaser light return path within the housing, onto the photosensor ofphoto-receiving circuit 12. Upon detection of this scan data signal,photoreceiving circuit 12 produces an analog scan data signal D₁indicative of the detected light intensity.

In the illustrative embodiment, photoreceiving circuit 12 generallycomprises scan data collection optics 53, which focus optical scan datasignals for subsequent detection by a photoreceiver 54 having, mountedin front of its sensor, a wavelength-selective filter 150 which onlytransmits optical radiation of wavelengths up to a small band above 670nanometers, as illustrated in FIG. 3. Photoreceiver 54, in turn,produces an analog signal which is subsequently amplified bypreamplifier 55 to produce analog scan data signal D₁. In combination,scanning means 11 and photoreceiving circuit 12 cooperate to generatescan data signals from the scan field, over time intervals specified bythe system controller. As illustrated hereinafter, these scan datasignals are used by bar code presence detection module 14, bar code scanrange detection module 15 and symbol decoding module 16.

As illustrated in FIG. 4, analog scan data signal D₁ is provided asinput to A/D conversion circuit 13. As is well known in the art, A/Dconversion circuit 13 processes analog scan data signal D₁ to provide adigital scan data signal D₂ which resembles, in form, a pulse widthmodulated signal, where logical "1" signal levels represent spaces ofthe scanned bar code and logical "0" signal levels represent bars of thescanned bar code. A/D conversion circuit 13 can be realized by anyconventional A/D chip. Digitized scan data signal D₂ is provided asinput to bar code presence detection module 14, bar code scan rangedetection module 15 and symbol decoding module 16.

The purpose and function of bar code presence detection module 14 is todetermine whether a bar code is present in or absent from the scan fieldover time intervals specified by the system controller. When a bar codesymbol is detected in the scan field, the bar code presence detectionmodule 14 automatically generates second control activation signal A₂(i.e., A₂ =1) which is provided as input to the system controller, asshown in FIG. 4. Preferably, bar code presence detection module 14 isrealized as a mircrocode program carried out by the microprocessor andassociated program and buffer memory, described hereinbefore. Thefunction of the bar code presence detection module is not to carry out adecoding process but rather to simply and rapidly determine whether thereceived scan data signals produced during bar code presence detection,represent a bar code symbol residing within the scan field. There aremany ways in which to realize this function through a programmingimplementation.

When a bar code symbol envelope is detected, the bar code symbolpresence detection module provides second control activation signal A₂=1 to the system controller. As will be described in greater detailhereinafter, second control activation signal A₂ =1 causes the device toundergo a transition from the bar code presence detection state to barcode symbol reading state.

The function of symbol decoding module 16 is to process, scan line byscan line, the stream of digitized scan data D₂, in an attempt to decodea valid bar code symbol within a predetermined time period allowed bythe system controller. When the symbol decoding module successfullydecodes a bar code symbol within the predetermined time period, symbolcharacter data D₃ (typically in ASCIII code format) is producedcorresponding to the decoded bar code symbol. Thereupon a third controlactivation signal A₃ is automatically produced by the symbol decodingmodule and is provided to the system controller in order to perform itssystem control function.

As shown in FIG. 4, system controller 22 generates and provides enablesignals E_(FC), E_(DS), E_(DT), to data format conversion module 17,data storage unit 18 and data transmission circuit 19, respectively, atparticular stages of its control program. As illustrated, symboldecoding module 16 provides symbol character data D₃ to data formatmodule 17 to convert data D₃ into two differently formatted types ofsymbol character data, namely D₄ and D₅. Format-converted symbolcharacter data D₄ is of the "packed data" format, particularly adaptedfor efficient storage in data storage unit 18. Format-converted symbolcharacter data D₅ is particularly adapted from data transmission to datacollection and storage device 3, or a host device such as, a computer orelectronic cash register. When symbol character data D₄ is to beconverted into the format of the user's choice (based on a selectedoption mode), the system controller will generate and provide enablesignal E_(DS) to data storage unit 18, as shown in FIG. 4. Similarly,when format converted data D₅ is to be transmitted to a host device, thesystem controller will generate and provide enable signal E_(DT) to datatransmission circuit 19. Thereupon, data transmission circuit 19transmits format-converted symbol character data D₅ to data collectiondevice 3, via the data transmission lines of flexible connector cable 4.

It is understood that there are a variety of ways in which to configurethe above-described system components within the housing of bar codesymbol reading device 2, while successfully carrying out the functionsof the present invention. In FIGS. 2 and 2A, one preferred arrangementis illustrated.

In FIG. 2A, the optical arrangement of the system components is shown.Specifically, visible laser diode 47 is mounted in the rear corner ofcircuit board 64 installed within the head portion of the housing. Astationary concave mirror 53 is mounted centrally at the front end ofcircuit board 63, primarily for collecting laser light. Notably, theheight of concave mirror 53 is such that it does not block lighttransmission aperture 6. Mounted off center onto the surface of concavemirror 53, is very small second mirror 64 for directing the laser beamto planar mirror 49 which is connected to the motor shaft of a scanningmotor 50, for joint oscillatory movement therewith. As shown, scanningmotor 50 is mounted centrally at the rear end of circuit board 63. Inthe opposite rear corner of circuit board 63, photodetector 54 ismounted.

In operation, laser diode 47 adjacent the rear of the head portion,produces and directs a laser beam in a forward direction to the smallstationary mirror 64 and is reflected back to oscillating mirror 49.Oscillating mirror 49 scans the laser beam over the scan field. Thereturning laser light, reflected from the bar code, is directed back tooscillating mirror 49, which also acts as a collecting mirror. Thisoscillating mirror then directs the beam to stationary concave mirror 53at the forward end of the housing head portion. The beam reflected fromthe concave mirror 53 is directed to photodetector 54 to produce anelectrical signal representative of the intensity of the reflectedlight.

In front of stationary concave mirror 53, IR LED 28 and photodiode 31are mounted to circuit board 63 in a slightly offset manner fromlongitudinal axis 9 of the head portion of the housing. Apertures 65 and66 are formed in opaque portion 7B of the housing below the transmissionaperture, to permit transmission and reception of IR type object sensingenergy, as hereinbefore described. In order to shield IR radiation fromimpinging on photodiode 31 via the housing, a metallic optical tube 67having an aperture 68 encases photodiode 31. By selecting the size ofaperture, the placement of photodiode 31 within optical tube 67 and/orthe radiation response characteristics of the photodiode, desiregeometric characteristics for the object detection field can beachieved, as described hereinbefore.

To prevent optical radiation slightly below 670 nanometers from enteringthe transmission aperture 6, and transmitting therethrough only opticalradiation from slightly below 670 nanometers, a plastic filter lens 69is installed over the transmission aperture 6, as shown in FIG. 1. Inthis way the combination of plastic filter lens 69 installed at thetransmission aperture and the wavelength selective filter 150 mountedbefore photoreceiver 54, as shown in FIG. 2A, cooperate with each otherin terms of wavelength selection characteristics, to form a narrowband-pass optical filter system having a center wavelength λ_(c) =670nanometers, as shown in FIG. 3.

In the illustrative embodiment, plastic window filter lens 69 is madefrom acrylic-type plastic material (e.g., DuPont RD 2177) which can bepurchased in 4'×8' sheets. These acrylic sheets are cut to size so as tofit over the light transmission aperture 6. The resulting plastic filterlens 69 is then installed into the light transmission aperture in amanner well known in the art.

Wavelength-selective filter 150 is preferably made by coating (i.e.,depositing) a multi-layer dielectric film onto a glass substrate. In avacuum environment (i.e., chamber), the dielectric film is preferablydeposited onto the glass substrate by evaporating a dielectric materialwith an electric beam, in a manner well known in the art. Thereafter,the resulting substrate with the dielectric film deposited thereon iscut into small pieces having physical dimensions approximately the sizeof the photosensor in photoreceiver 2, as shown in FIGS. 2 and 2A,thereby providing wavelength-selective filter 150. Thewavelength-selective filter 150 is then mounted immediately in front ofthe photosensor, as shown in FIGS. 2 and 2A.

The novel optical filter arrangement described above provides a numberof important advantages to the laser scanner in which it is embodied.

Firstly, the narrow-band optical filter system of the present inventionrejects wavelengths outside the narrow-band of spectral componentscomprising the laser scanning beam (i.e. associated with ambient lightnoise), and this improves the signal-to-noise ratio for detected scandata signals D₁.

Secondly, the spectral filtering characteristics of plastic filter lens69 inherently appears reddish to the human vision system by virtue ofthe fact that lens 69 only permits transmission of optical radiationfrom slightly below 670 nanometers. Thus, the semi-transparent nature offilter lens 69 naturally hides from plain view, the laser, the mirror,the scanning motor, and other electro-optical components within thehousing that otherwise might present source of fear in customers at aPOS station, and/or detract from the aesthetic appearance of thescanning system installed at POS station.

Thirdly, the plastic filter lens 69 with its specified opticalproperties is easy and inexpensive to manufacture using injectionmolding techniques well known in the art. Thus, it may be made as largeas desired or formed (i.e., shaped) to embody beam-shaping orbeam-directing characteristics, without substantially increasing thecost of manufacture of this optical filter element.

Wavelength-selective filter element 150, on the other hand, is veryexpensive and difficult to manufacture, by virtue of its specifiedoptical properties. However, as this optical filter element 150 isinstalled along the focused laser light return path, in front ofphotoreceiving sensor 54 as shown in FIG. 2A, its size can be maintainedextremely small, independent of the surface area of the lighttransmission aperture, and thus the plastic filter lens 69.Consequently, conventional techniques can be used to manufacture thissmall-sized optical filter element, and thus the cost of manufacture ofthis optical element can be minimized.

Fourthly, by using spatially-separated optical filter elements (i.e.,plastic filter lens 69 and filter element 150), the use of specialoptical cements and bonding techniques otherwise required to physicallybound such elements together in an integral filter structure, areavoided altogether. This fact simplifies significantly themanufacturability of the laser scanner of the present invention.

The optical filter system described above may be embodied in any type oflaser bar code symbol scanner. An example of such an alternative laserscanner design is shown in FIGS. 5 and 5A.

In FIGS. 5 and 5A, the optical filter system of the present invention isshown embodied in the laser projection scanner disclosed in U.S. Pat.No. 5,216,232. As disclosed in FIGS. 5 and 5A, plastic filter element69' is functionally similar to optical filter element 69 and covers thelight transmission aperture of the compact housing of the laserprojection scanner, while wavelength selective filter 150' is disposedin front of its photodector 110 along the focused laser light returnpath defined between light focusing mirror 120 and photodector 110, asshown in FIG. 5A. By virtue of the principles of the present invention,plastic filter element 69' over light transmission aperture 6' can bemade substantially larger than wavelength selective filter 150', asrequired in practical scanner designs, yet it provides all of theadvantages described above.

In alternative laser scanner designs the alternate optical filter systemdisclosed herein may be embodied within laser holographic scanners usedto read code symbols in various applications.

While the particular illustrative embodiments shown and described abovewill be useful in many applications in code symbol reading, furthermodifications to the present invention herein disclosed will occur topersons skilled in the art. All such modifications are deemed to bewithin the scope and spirit of the present invention defined by theamended claims.

What is claimed is:
 1. A laser symbol scanning system, comprising:ahousing having a light transmission aperture through which light canenter and exit said housing; a first optical filter element disposed insaid light transmission aperture and along a laser light path extendingthrough said light transmission aperture, and havingwavelength-selective filtering characteristics, said first opticalfilter element function as a scanning window in said housing; laser beamproducing means in said housing for producing a laser beam characterizedby a predetermined wavelength; laser beam scanning means in said housingfor projecting said laser beam through said scanning window and scanningsaid laser beam across a code symbol on an object located within atleast a portion of a scan field defined external to said housing; laserlight detection means is said housing disposed in said laser light path,for detecting the intensity of laser light reflected off said codesymbol; and a second optical filter element in said housing, spatiallyseparated from said first optical filter element and disposed along saidlaser light path between said first optical filter element and saidlaser light detection means, and having wavelength-selective filteringcharacteristics, said second optical filter element cooperating withsaid first optical filter element so as to form a band-pass opticalfiltering system having a narrow wavelength bandwidth positioned aboutsaid predetermined wavelength and passing light reflected off said codesymbol having wavelengths only within said narrow wavelength bandwidth.2. The laser code symbol scanning system of claim 1, wherein said firstoptical filtering element prevents light having wavelengths up toslightly below said predetermined wavelength from passing through saidfirst filter.
 3. The laser code symbol scanning system of claim 1,wherein said second optical filtering element transmits light havingwavelengths from slightly above said predetermined wavelength.
 4. Thelaser code symbol scanning system of claim 1, wherein said predeterminedwavelength is about 670 nanometers.
 5. The laser code symbol scanningsystem of claim 2, wherein said predetermined wavelength is about 670nanometers.
 6. The laser code symbol scanning system of claim 3, whereinsaid predetermined wavelength is about 670 nanometers.
 7. The laser codesymbol scanning system of claim 1 wherein said laser beam producingmeans comprises a visible laser diode for producing a visible laserbeam.
 8. The laser code symbol scanning system of claim 1 wherein saidsecond optical filtering element is disposed immediately adjacent saidlight detecting means.
 9. The laser code symbol scanning system of claim1 wherein said wavelength filtering characteristics of said firstoptical filtering element obscures each of said means positioned in saidhousing from plain view.
 10. The laser code symbol scanning system ofclaim 1 wherein in said housing is a compact hand-supportable housing.11. The laser code symbol scanning system of claim 1 wherein saidhousing is mounted above a countertop.
 12. The laser code symbolscanning system of claim 1 wherein said laser beam scanning meansproduces a single-line laser scanning pattern.
 13. The laser code symbolscanning system of claim 1 wherein said laser beam scanning meansproduces an omnidirectional laser scanning pattern.